His cutie mark contains two lightning bolts going in a circle with the Wardenclyffe Tower behind them. This was supposed to symbolize his great success' and contribution to science as well as his grand dreams that never came to fruition (the lightning bolts representing alternating current and the tower was one of his projects that ran out of money before its completion).

Here you can find a version without the accessories (Tesla coil, cloud etc) and here's one without the clothes.

Thanks to everyone who suggested him!

PSD file. As explained in my profile, you are free to use this image for any purpose:

This is the ultimate pony. Chrysalis is trying to take over the world? Tesla coil. Diamond dogs? Tesla coil. Nightmare Moon? Really big Tesla coil. Discord? Meh, those six ponies from Ponyville could probably handle it, I need to make a BIGGER Tesla coil.

It's funny how one can always find Pony stuff, even when searching for formerly obscure scientists.

Also, if you'd like know, Tesla's eyes were grey-blue, which was unusual for a Serb. He claimed it was because his constant thinking turned them that color, but this was probably a joke; his mother also had grey eyes.

It would be more symbolic if the little tower in his hoof has no wire - there were stories about wireless energy (and the tower in his cutie mark is the grand finale of it, being the biggest wireless energy provider of all time - you could hold a Glowing light bulb in your hands. The project ran out of money because the wireless energy cant be controlled by companies (= no profit) )Oh well, goes to my favs anyway